diff --git a/containers.mllib b/containers.mllib
index 0f0679f1..990ac5ee 100644
--- a/containers.mllib
+++ b/containers.mllib
@@ -21,3 +21,4 @@ PiCalculus
 Bencode
 Sexp
 RAL
+MultiSet
diff --git a/containers.odocl b/containers.odocl
index 0f0679f1..990ac5ee 100644
--- a/containers.odocl
+++ b/containers.odocl
@@ -21,3 +21,4 @@ PiCalculus
 Bencode
 Sexp
 RAL
+MultiSet
diff --git a/multiSet.ml b/multiSet.ml
new file mode 100644
index 00000000..b834d5c4
--- /dev/null
+++ b/multiSet.ml
@@ -0,0 +1,138 @@
+(*
+copyright (c) 2013, simon cruanes
+all rights reserved.
+
+redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+this software is provided by the copyright holders and contributors "as is" and
+any express or implied warranties, including, but not limited to, the implied
+warranties of merchantability and fitness for a particular purpose are
+disclaimed. in no event shall the copyright holder or contributors be liable
+for any direct, indirect, incidental, special, exemplary, or consequential
+  damages (including, but not limited to, procurement of substitute goods or
+  services; loss of use, data, or profits; or business interruption) however
+  caused and on any theory of liability, whether in contract, strict liability,
+  or tort (including negligence or otherwise) arising in any way out of the use
+  of this software, even if advised of the possibility of such damage.
+*)
+
+(** {1 Multiset} *)
+
+module type S = sig
+  type elt
+  type t
+
+  val empty : t
+
+  val mem : t -> elt -> bool
+
+  val count : t -> elt -> int
+
+  val singleton : elt -> t
+
+  val add : t -> elt -> t
+
+  val remove : t -> elt -> t
+
+  val min : t -> elt
+
+  val max : t -> elt
+
+  val union : t -> t -> t
+
+  val intersection : t -> t -> t
+
+  val diff : t -> t -> t
+
+  val compare : t -> t -> int
+
+  val equal : t -> t -> bool
+
+  val iter : t -> (int -> elt -> unit) -> unit
+
+  val fold : t -> 'b -> ('b -> int -> elt -> 'b) -> 'b
+end
+
+module Make(O : Set.OrderedType) = struct
+  module M = Map.Make(O)
+
+  type t = int M.t
+
+  type elt = O.t
+
+  let empty = M.empty
+
+  let mem ms x = M.mem x ms
+
+  let count ms x =
+    try M.find x ms
+    with Not_found -> 0
+
+  let singleton x = M.singleton x 1
+
+  let add ms x =
+    let n = count ms x in
+    M.add x (n+1) ms
+
+  let remove ms x =
+    let n = count ms x in
+    match n with
+    | 0 -> ms
+    | 1 -> M.remove x ms
+    | _ -> M.add x (n-1) ms
+
+  let min ms =
+    fst (M.min_binding ms)
+
+  let max ms =
+    fst (M.max_binding ms)
+
+  let union m1 m2 =
+    M.merge
+      (fun x n1 n2 -> match n1, n2 with
+        | None, None -> assert false
+        | Some n1, None -> Some n1
+        | None, Some n2 -> Some n2
+        | Some n1, Some n2 -> Some (n1+n2))
+      m1 m2
+
+  let intersection m1 m2 =
+    M.merge
+      (fun x n1 n2 -> match n1, n2 with
+        | None, None -> assert false
+        | Some _, None
+        | None, Some _ -> None
+        | Some n1, Some n2 -> Some (Pervasives.min n1 n2))
+      m1 m2
+
+  let diff m1 m2 =
+    M.merge
+      (fun x n1 n2 -> match n1, n2 with
+        | None, None -> assert false
+        | Some n1, None -> Some n1
+        | None, Some n2 -> None
+        | Some n1, Some n2 ->
+          if n1 > n2 
+            then Some (n1 - n2)
+            else None)
+      m1 m2
+
+  let compare m1 m2 =
+    M.compare (fun x y -> x - y) m1 m2
+
+  let equal m1 m2 =
+    M.equal (fun x y -> x = y) m1 m2
+
+  let iter m f =
+    M.iter (fun x n -> f n x) m
+
+  let fold m acc f =
+    M.fold (fun x n acc -> f acc n x) m acc
+end
diff --git a/multiSet.mli b/multiSet.mli
new file mode 100644
index 00000000..189997e3
--- /dev/null
+++ b/multiSet.mli
@@ -0,0 +1,63 @@
+(*
+copyright (c) 2013, simon cruanes
+all rights reserved.
+
+redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+redistributions of source code must retain the above copyright notice, this
+list of conditions and the following disclaimer.  redistributions in binary
+form must reproduce the above copyright notice, this list of conditions and the
+following disclaimer in the documentation and/or other materials provided with
+the distribution.
+
+this software is provided by the copyright holders and contributors "as is" and
+any express or implied warranties, including, but not limited to, the implied
+warranties of merchantability and fitness for a particular purpose are
+disclaimed. in no event shall the copyright holder or contributors be liable
+for any direct, indirect, incidental, special, exemplary, or consequential
+  damages (including, but not limited to, procurement of substitute goods or
+  services; loss of use, data, or profits; or business interruption) however
+  caused and on any theory of liability, whether in contract, strict liability,
+  or tort (including negligence or otherwise) arising in any way out of the use
+  of this software, even if advised of the possibility of such damage.
+*)
+
+(** {1 Multiset} *)
+
+module type S = sig
+  type elt
+  type t
+
+  val empty : t
+
+  val mem : t -> elt -> bool
+
+  val count : t -> elt -> int
+
+  val singleton : elt -> t
+
+  val add : t -> elt -> t
+
+  val remove : t -> elt -> t
+
+  val min : t -> elt
+
+  val max : t -> elt
+
+  val union : t -> t -> t
+
+  val intersection : t -> t -> t
+
+  val diff : t -> t -> t
+
+  val compare : t -> t -> int
+
+  val equal : t -> t -> bool
+
+  val iter : t -> (int -> elt -> unit) -> unit
+
+  val fold : t -> 'b -> ('b -> int -> elt -> 'b) -> 'b
+end
+
+module Make(O : Set.OrderedType) : S with type elt = O.t